Surface acoustic wave devices are generally manufactured through forming metal layers over the surface of a piezoelectric substrate of a wafer form by vapor deposition, coating the metal layers with a resist, subjecting them to exposure and development process, dry etching the metal layers to shape a desired pattern of an electrode, and slicing them into chips.
Such a method allows the electrode to have a rectangular shape in its cross section. Each side surface of the cross section extends substantially perpendicular to the piezoelectric substrate as is configured sharply. Having its side surfaces being sharp, the electrode may reflect surface acoustic waves, thus hardly providing a desired frequency response.
FIG. 14 illustrates a conventional surface acoustic wave device 140 disclosed in Japanese Patent Laid-Open Publication No. 9-46168 for overcoming the above drawback. An electrode 142 on a piezoelectric substrate 141 has a moderate shape in its cross section having each side surface rounded and can thus attenuate the reflection of surface acoustic waves at the side surface, hence ensuring a desired frequency response.
A propagation frequency of a surface acoustic wave device is generally determined by the distance between comb electrodes and by the thickness of electrode layers. The comb electrodes can have the distance and the thickness accurately by a photo-lithography technique.
If having an excessively-large thickness or material density with the distance remaining uniform, the electrode provides a low propagation frequency. More particularly, if being heavy, the electrode may interrupt oscillation of the piezoelectric substrate, hence lowering the frequency.
A device having the electrode having a cross section having the round shape in a direction perpendicular to the piezoelectric substrate can suppress the reflection of surface acoustic waves on its sides. However, the shape cannot be determined, thus providing variations in its shape, size, and mass. The propagation frequency of the conventional surface acoustic wave device hardly be uniform accordingly.
Also, If the electrodes are contaminated at the surfaces with electrically conductive impurities during a surface mounting process or a packaging process of the device, the conventional surface acoustic device may be declined in the properties and short-circuited.
FIG. 15 illustrates another surface acoustic wave device for overcoming the above drawback as disclosed in Japanese Patent Laid-Open Publication No. 9-153755. An electrode 102 is formed on a piezoelectric substrate 101, and the electrode and a surface of the piezoelectric substrate 101 are covered with an insulating layer 103 together. The insulating layer 103 can prevent the surface of the surface acoustic wave device from short-circuit and electrically conductive impurities.
However, when the insulating layer 103 repeats to be pulled and pushed due to thermal expansion and compression, such as heat cycle, the layer may receive a significant stress and deteriorate due to a difference between respective thermal expansion coefficients of materials adjoining each other. Since the insulating layer 103 simply covers the electrodes 102 and the piezoelectric substrate 101, a bonding strength to the electrodes 102 and the piezoelectric substrate 101 may easily decline and be detached, hence resulting in the deterioration or short-circuit.
If the insulating layer 103 is thick for improving its physical strength, overall loss in the conventional surface acoustic wave device increases. If the layer 103 is thin, the insulating layer 103 may be detached while the loss in the conventional surface acoustic wave device is small.